Air drying systems are well known and practiced in a variety of technical fields. One such prior art air drying system is a single tower system disclosed in U.S. Pat. No. 5,423,129. Disclosed therein is a single tower air drying system designed to provide clean and dry compressed air to a pneumatic system such as a brake system of a railroad train. That prior art system accomplishes this by removing moisture and airborne particulates from a stream of compressed air as it passes through a desiccant material contained within a single tower. FIG. 1 of that patent illustrates a cross-sectional view of the patented system. From right to left FIG. 1 of that patent shows an opening through which unpurified compressed air is received; a sump volume; the single tower housing the desiccant material; a purge check valve with a choke; a side chamber connected to a purge volume; a discharge air filter element; a discharge check valve; and an output chamber through which purified compressed air passes eventually to the pneumatic system.
In operation, the patented air drying system receives a supply of compressed air from an air compressor which typically contains an unacceptably high amount of moisture and other particulates suspended within the compressed air. This unpurified compressed air passes into the sump volume and then flows upwardly eventually reaching the desiccant material. The desiccant plays the key role within the single tower air drying system in that it absorbs the moisture and traps various particulates (e.g., dust, dirt, etc.) as the compressed air moves radially into and through the desiccant material. Once moisture and particulates are extracted from the air stream, the cleaned and dried compressed air continues flowing from the center of the desiccant material through the purge check valve situated near the top of the single tower. This purified compressed air then passes through the side chamber eventually reaching the purge volume.
The purge volume of the subject patented air drying system is capable of holding approximately five-hundred cubic inches (500 in.sup.3) of purified compressed air. When the air compressor is cycled off, the single tower system operates in a purge mode. During the purge mode, the purified pressurized air contained within the purge volume passes slowly in the reverse direction through the choke in the purge check valve and then back through the desiccant material. This slow stream of dried air reabsorbs a portion of the moisture previously collected within the desiccant material. Having evaporated in this passing stream of dry air, the evaporated moisture eventually exhausts through the sump volume to atmosphere. This gradual purging of dry air back through the system serves to dry-out and thus rejuvenate the desiccant material. When the air compressor is again cycled on, the single tower system operates in a drying mode. During the drying mode, the desiccant material then again removes moisture from the stream of unpurified compressed air passing therethrough.
More recently, a double tower system has been proposed and developed in which a pair of desiccant containing towers are provided, each alternating back and forth between operation in drying mode and in recycle mode. This unique system avoids the need to have the source of unpurified air cycled-off in order to purge the desiccant material of the moisture it has accumulated, and thereby eliminate temporarily depriving the pneumatic system of a steady supply of clean and dried compressed air while the compressor is turned off, and in addition, is not otherwise limited in water removing capacity.
Whether utilizing a single tower or a double tower, the check valves utilized to regulate the streams of air through the desiccant containing tower or towers are rather complex as valves go, and are manufactured to rather exacting dimensions to assure positive control of the air streams which normally are under significant compression pressure. Typically, abutting valve surfaces are machined to provide a carefully machined ring-like metallic protuberance or raised ring portion, which to close the valve, is compressed into a flat rubber, or otherwise resilient, seal surface. Accordingly, careful and costly machining is required to achieve the solid metallic, raised ring surface or protuberance which, in operation, will be abut against and compress into the flat resilient seal member.